This invention relates a method of making pairs of cooled thread split inserts used to injection mold bottle preforms.
As seen in the applicant""s U.S. Pat. No. 5,599,567 which issued Feb. 4, 1997, it is well known to use a pair of thread split inserts in a mold to form the threaded neck portion of a PET bottled preform. The neck portion of the preform also has a ring collar which is used to eject the preform from the mold. The thread split inserts have conduits through which cooling fluid is circulated to cool the neck portion of the preform prior to ejection.
In the past, thread split inserts have been made by machining steel upper and lower parts and then integrally brazing them together. This method has the disadvantage that both parts must be machined to provide cooling fluid conduits and threads and this is time consuming and therefore relatively costly.
Accordingly, it is an object of the present invention to at least partially overcome the disadvantages of the prior art by providing a method of making cooled thread split inserts wherein an inner part which fits in an outer part is made by casting rather than machining.
To this end, in one of its aspects, the invention provides a method of making a pair of thread split inserts used in injection molding elongated hollow bottle preforms. Each preform has a neck portion with an outer surface forming a ring collar and threads extending between an open end and the ring collar. Each thread split insert has a front end, a rear end and first and second flat inner aligned faces extending on opposite sides of a curved inner surface. The thread split inserts are mounted together in a mold with the respective flat inner faces of the thread split inserts abutting, wherein the curved inner surfaces of the thread split inserts combine to form an opening therethrough shaped to mold the outer surface of the neck portion of the preform. The curved inner surfaces of the thread split inserts each have a semicircular groove to form the ring collar and a threaded portion extending between the semicircular groove and the rear end to form the threads. The method comprises the steps of injection molding a ceramic core having a predetermined shape and then casting wax around the ceramic core in the shape of an inner part of the pair of thread split inserts. Then investment casting a suitable metal in a mold to replace the wax around the ceramic core to form a hollow inner part of the pair of thread split inserts extending around a central longitudinal axis. The inner part has a generally cylindrical outer surface with grooves therein to partially form inner portions of two cooling fluid conduits. Each cooling fluid conduit extends around the curved inner surface of one of the thread split inserts. Then machining the cast inner part to make the outer surface a predetermined size. Making a hollow outer part of the pair of thread split inserts of a suitable metal having a predetermined shape with an opening therethrough having an inner surface which fits around the outer surface of the inner part and outer portions of the two cooling conduits extending from the opening therethrough to respective inlets and outlets. Mounting the outer part around the inner part with the inner and outer portions of the two cooling fluid conduits aligned. Then applying brazing material between the inner part the outer part and heating the assembled inner part and outer part in a vacuum furnace to integrally braze the inner part and the outer part together. Cutting the integral inner and outer parts in half along the central longitudinal axis to form the pair of thread split inserts, with each of the thread split inserts having one of the cooling fluid conduits therein.
Further objects and advantages of the invention will appear from the following description taken together with the accompanying drawings.